My project involves characterizing the surface structure of magnetite, Fe3O4, (100), (110), and (111) terminations using crystal truncation rod (CTR) diffraction in a hydrated atmosphere along with developing a surface preparation method that results in a well-ordered, smooth surface. AFM images of smooth, prepared surfaces (110) and (111)

Email: fnscp@uaf.edu Sarah Petitto's CV

Magnetite crystallizes in an inverse spinel crystal lattice where the octahedral sites are occupied with both Fe²⁺ and Fe³⁺ cations, the tetrahedral sites are occupied with Fe³⁺ cations, and the O^2- anions packed in a face-centered cubic (FCC) lattice.

Pictures of ideal, bulk magnetite surface terminations (100) (110) (111):

Fe3O4 (100) top view

Fe3O4 (100) side view

Fe3O4 (110) top view

Fe3O4 (110) side view

Fe3O4 (111) top view

Fe3O4 (111) side view

Analysis of CTR diffraction data allows us to develop a 3-D surface structure model under environmentally relevant in-situ conditions, allowing us the opportunity to study the impact of surface hydroxylation and surface reduction/oxidation. Magnetite is common iron-oxide phase found in a range of soils and sediments and is an essential component in the heterogeneous reduction of aqueous contaminants and the Fe biogeochemical cycle. The surface reactivity of metal oxides in the environment can result in hydroxyl proton exchange, surface complexation of aqueous solutes, and surface catalyzed (or induced) redox reactions, which are dependent on composition, local coordination, and topographical arrangement of atoms. I study model surface substrates (single crystals) to help identify these driving forces controlling surface structure, surface reactivity, and terminating surface moieties in order to understand the surface reactions that occur on natural samples, which govern the composition of natural waters and in regulating the transport and bioavailability of contaminants in aquatic systems.

Preliminary fitted CTR rods of Fe₃O₄ (111): (00L), (10L), and (31L):

Fe3O4 (111) (00L)

Fe3O4 (111) (10L)

Fe₃O₄ (111) (31L)

My project also is focused on controlling surface terminations using an electrochemical cell in order to drive the surface to a reduced or oxidized state. Magnetite is an ideal substrate for controlled redox reactions with both Fe²⁺/Fe³⁺ cations present. The redox properties, which are environmental relevant due to its ability to effectively reduce aqueous species including Fe(III), Cr(VI) and halogenated organics. Redox controlled surfaces will allow us to further understand a variety of processes including film growth, adsorption, mass-transport, roughness development at interfaces, and guidance in designing new electrochemical devices and experiments. The surface reactivity of these oxidized/reduced surfaces will be examined using aqueous species uptake to emulate the contaminants in aquatic systems. I am currently working with our collaborators on designing an electrochemical cell that can be used with the CTR diffractometer, which will allow use to stabilize the surface in a specific configuration either oxidized or reduced by adjusting the applied potential and solution composition (no dissolved O₂, fixed pH, or the addition of species (Fe(II)/Fe(III)), and will potentially allow us to heal the surface from defects and vacancies.

A complimentary set of experiments is measuring in-situ ion distributions in the electrical double layer using X-ray reflectivity. The electrical double layer (EDL) is the distribution of charge that develops at the interface between an electrolyte and a charged solid surface shown as a large density contrast between EDL and substrate atoms. This phenomenon occurs in many electrochemical systems including colloids, biological membranes, electrode/electrolyte interfaces, and mineral/fluid interfaces. The EDL is typically envisioned as consisting of a discreet layer of ions adsorbed to the charged substrate surface and a diffuse distribution of ions that balance the net surface charge and are governed by the decay in electrostatic potential with distance from the charged interface. An electrochemical cell for this experimental set-up also is being designed to monitor and control changes in reflectivity signal using various assumed charge distributions at the electrode/electrolyte interface.

Schematic of EDL experiment